This invention relates to a film-folding device for a packaging apparatus, and, more particularly, to such a device for a weighing-packaging-pricing apparatus for weighing an item such as fresh produce placed on a tray, packaging it by wrapping it together with the tray with a stretch sheet of polyethylene film or the like, and issuing a printed price label showing its price.
So-called pre-packaged merchandises, such as fresh produce placed on a styrofoam tray and wrapped with a stretch film with the tray, are commonly available nowadays at supermarkets and elsewhere. Apparatus for producing such pre-packaged merchandises usually includes a roll supporting device for rotatably supporting a roll of stretch film, a film transporting device for providing a film sheet of a desired size and holding it stretched for a packaging process, a weighing device for weighing items to be packaged including a tray, a lifter for lifting the weighed item to press it against a film sheet stretched above the lifter, a folding machine for folding the edge sections of the film sheet around the edges of the tray, and a sealing device for heating the folded edge sections of the film sheet. In addition, some apparatus are also provided with a label printer for printing the price of each packaged product on a label to be pasted thereon. It now goes without saying that each of these many constituent devices, having different functions to perform, can be improved.
Regarding the mechanism for supporting a roll of film, it is desirable that such a mechanism be able to support film rolls of different lengths (for films of different widths) so that trays of different sizes can be used. According to Japanese Patent Publication Tokkai 57-194915, a plurality of mutually parallel and vertically extending endless belts for clamping one side edge of a film to be transported are stretched around a frame. A bar is attached to this frame and the user holds this handle to manually move the frame in the transverse direction according to the width of the film to be used. When a different film roll with a different length is to be used, however, at least one of the film transporting mechanisms on one side of the film must be manually moved in the transverse direction so as to adjust the distance between the mechanisms for clamping the individual side edge sections of the film according to the length of the new film roll to be installed. Such adjustments are extremely cumbersome.
After a film is pulled out of such a roll, it is directed to a film-feeding mechanism which usually includes a motor-operated film-feeding roller and a holding member contacting this roller. The front edge of the film is hung around the roller so as to be sandwiched between this roller and the holding member. In order to make certain that the film will advance reliably when this roller is driven by a motor, use is generally made of a roller with a high-precision mirror surface in order to increase friction between the roller surface and the film. The film, on the other hand, tends to stick to this mirror surface. When a new roll of film is installed, therefore, the user finds it extremely difficult to quickly and accurately direct its front edge to the film-feeding mechanism. In other words, the user usually lifts the holding member and inserts the film into the film-feeding mechanism while keeping the space above the film-feeding roller empty but this space is limited and it is difficult to prevent the film from coming into contact with this mirror surface. If even a small portion of the film comes into contact with the mirror surface and becomes firmly attached to it, the film cannot be advanced evenly as a whole and tends to become wrinkled. Moreover, when this film-feeding roller is not being operated by its motor, the mot or serves as a brake and film-feeding roller cannot be rotated easily by hand. This makes it also difficult to adjust the orientation of the film once it becomes attached to the mirror surface of the film-feeding roller.
If trays of different sizes are used but the film is always cut into sheets of the same size, the tray may be too big and cannot be properly packaged or may be too small and the edge sections to be folded are too wide. In order to adjust the size of the sheets into which the film is cut according to the size of the tray to be used, Japanese Patent Publication Tokkai 61-81913 disclosed a method whereby a plurality of sensors are provided along the path on which trays are transported. Use of a large number of sensors, however, increases the production cost of the apparatus.
As for the lifter for lifting a weighed object up to a position where it is packaged, use is usually made of a plurality of legs supported on a base plate for the convenience of allowing folding plates to slide underneath the tray when the edge sections of the film sheet are folded downward towards the bottom surface of the tray. Each leg is capped by a head which can be bent in a specified direction to accommodate these folding plates. If a lifter with a large base plate and a large number of legs is used to support and lift a tray which is relatively small, however, those of the legs in peripheral areas not supporting the tray tend to obstruct the motion of the folding plates and/or to prevent the edge sections of the film from hanging down sufficiently low so as to be properly folded by the folding plates. Japanese Patent Publication Tokkai 63-248612 disclosed a lifter with two groups of legs, a center group for use in the case of a smaller tray and a peripheral group for use together with the center group in the case of a larger tray. According to this disclosure, however, two lifter base plates are required, each provided with its own driving means for its vertical motion. As a result, the overall structure becomes complicated and the apparatus becomes large.
As for the folding device, it is usually provided with a pair of left-hand and right-hand side folding plates and a back folding plate which move transversely and longitudinally, respectively, to fold the edge sections of the film sheet stretched over the loaded tray. A discharge pusher is then operated to push the tray forward past a perpendicularly extending bar by means of which the front edge section of the film sheet is also folded towards the bottom surface of the tray. Needless to say, these folding plates and the discharge pusher must be operated in a well coordinated manner according to a carefully designed time schedule. Japanese Patent Publications Tokkai 61-60410 and 61-190408 disclosed systems whereby the timing for the operation of the left-hand and right-hand side folding plates can be varied according to the transverse (left-to-right) dimension of the tray to be packaged. In other words, the longitudinal dimension of the tray is completely ignored and its folding plates are operated without regard to it. Thus, the folded film sheet may become wrinkled or slackened especially near the corners. When a loaded tray is lifted against a stretched film sheet, the film sheet should be properly stretched not only in the transverse direction but also in the longitudinal direction (the direction of its motion). Japanese Patent Publication Tokkai 63-71060 suggested a use of a dancer roller between the film roll and the film-supplying system but the position of such a roller must be accurately determined in order to provide a uniform tension to the film sheet. Moreover, the film is bent around such a roller and hence its setting becomes complicated and workability of the apparatus is adversely affected. In the case of an apparatus with a cutter which cuts the film into a desired size before it is used for packaging, furthermore, the tension provided by the dancer roller has no effect on the tension in the cut sheet. Japanese Utility Model Publication Jikkai 61-48106 disclosed a pair of units disposed for clamping a film to transport it in such a way that their separation increases in the direction of film transportation. In this case, however, the tension on the film is not uniform along the direction of its motion. The tension on one side may become excessively large or the other side may not experience enough tension. If the tension is too large, peripheral parts of the tray may be bent or the film may break. If the tension is insufficient, the film is likely to develop wrinkles or slack parts. With a film transporting device of this type, furthermore, the clamps on the side edge sections of the film being transported should not be released too soon or too late. If the clamps are released too soon, the film becomes slack. If the clamps are released too late, an excessive tension may be applied on the film when the folding plates push the edge sections of the film, causing the film to break.
When an apparatus for weighing and packaging is used only for the purpose of weighing already packaged products and of having price labels printed, the apparatus is operated in a different mode and the sealing device, which is not required in such mode of operation, may obstruct the user's operation. Moreover, it is cumbersome to input the selection of a different mode of operation every time it is desired to use the apparatus only for having price labels printed.